1. Field of the Invention
This invention relates generally to semiconductor devices, and more particularly to bipolar junction transistors.
2. Description of the Prior Art
As known in the art, bipolar junction transistors (BJTs), which can be formed using a CMOS compatible process, are key parts of analog integrated circuits such as band-gap voltage reference circuits. These circuits are often sensitive to Vbe (base-emitter voltage) value and Vbe mismatch of BJT.
Unfortunately, the prior art CMOS process compatible BJT structure is not able to control Vbe value and the Vbe mismatch characteristic is unsatisfactory due to salicide non-uniformity that typically occurs at the edge of the active region. The salicide is formed in the active region to reduce the contact resistance. It has been found that salicide encroachment at the edge of the active region causes P+/N well junction leakage, thus leading to worse Vbe mismatch performance. One approach to improve salicide non-uniformity is to reduce cobalt thickness during the salicide formation. However, this approach adversely affects resistance for non-salicide resistors.
Therefore, there is a need in the industry to provide an improved structure of bipolar junction transistors, which is able to control Vbe value of the BJT and provide reduced Vbe mismatch.